This invention relates to quadrature radio frequency coil design for use in magnetic resonance experiments.
One type of magnet used in magnetic resonance imaging is that which provides a cylindrical bore in which the main field of the magnet is axial along the length of the bore. Magnets of this type provide a high magnetic field that is more forgiving to inefficiencies in the design and use of other components in the system, without reducing the signal to noise ratio beyond an acceptable limit.
For the purposes of carrying out various experiments it is desirable to provide a volume coil in which the sample, which may in some cases be a part of the body of a patient, can be inserted into a space inside the coil that provides a substantially homogeneous RF field throughout the anatomy of interest.
Images can be obtained from such magnets using several common types of coil to generate radio frequency fields, including the known arrangements of the Alderman-Grant, saddle or Helmholtz coil designs. The radio frequency fields produced by the above coils are however linearly polarized.
By using circularly polarized fields (from so called xe2x80x9cquadraturexe2x80x9d coils), a theoretical improvement of up to 40% in signal to noise ratio is possible. Many designs of quadrature type coils are available, however all or most are designed for an arrangement using the above cylindrical bore type magnet in which the main field of the magnet is longitudinal with the patient.
The common cylindrical magnet provides a high magnetic field strength and therefore higher image resolution, but has significant disadvantages in that it limits access to the patient and in that it generally requires superconducting elements and therefore cryogenics in order to obtain the higher field strength.
Other magnet designs in use are of a low field, xe2x80x9copenxe2x80x9d type in which the main magnetic field is perpendicular to the axis of the patient, and Is substantially vertical. These magnets have advantages of improved access and can in many cases avoid the necessity for cryogenics by the use of permanent magnets. However they produce a significantly reduced magnetic field so it is necessary to ensure high efficiency in the use and arrangement of the RF coil to overcome the loss of resolution inherent in low field designs.
Prior art coils are shown in U.S. Pat. No. 5,483,159 (Van Heelsvergen) issued Jan. 9, 1996; U.S. Pat. No. 5,057,777 (Kurczewski) issued October 1991; U.S. Pat. No. 4,875,013 (Murakami) issued Oct. 17, 1989 and U.S. Pat. No. 5,445,153 (Sugie) issued Aug. 29, 1995.
Van Heelsvergen, Kurczewski and Sugie disclose designs of orthogonal or quadrature coils for use in magnetic resonance imaging. However none of these designs is suitable for an arrangement as set forth above in which the magnetic field is at right angles to the direction of insertion of the part of the patient or sample into the hollow interior of the coil.
Van Heelsvergen discloses a coil formed of two spaced, parallel, coaxial, annular coil portions interconnected at diametrically opposed positions by conductors extending at right angles to the planes of the two annular coil portions. However the two modes of resonance are produced by current paths which are entirely different from those of the present invention and the design is unsuitable for the arrangement as set forth in the present invention.
It is one object of the present invention to provide a quadrature radio frequency coil for use in a magnet in which the magnetic field lies at right angles to a direction of insertion of a sample into the magnetic field. Such a coil is particularly but not exclusively arranged for use with a magnet where the field is vertical and the sample is inserted horizontally.
The present invention, according to a first aspect, provides a quadrature radio frequency field coil for use in a unidirectional magnetic field for effecting magnetic resonance experiments on a sample comprising:
a first annular conductor lying generally in a first plane at a first end of the coil;
a second annular conductor lying generally in a second plane at a second end of the coil;
the first and second planes being parallel and spaced so as to define a hollow interior of the coil into which the sample can be received;
the first annular conductor including at least one first capacitor connected therein in series;
the second annular conductor including at least one second capacitor connected therein in series;
the first and second annular conductors and the first and second capacitors being arranged such that, in a first mode, current can be caused to flow therein to generate a first oscillating radio frequency field, which is substantially homogeneous within the hollow interior, at a resonant frequency dependent upon the shape and inductance of the first and second annular conductors, the capacitance, and the mutual inductance therebetween;
a first plate conductor having a length extending across the annular conductors generally at right angles to the first and second planes so as to bridge the space therebetween;
a second plate conductor having a length extending across the annular conductors generally at right angles to the first and second planes so as to bridge the space therebetween;
the first and second plate conductors being arranged at diametrically opposed locations on the annular conductors and being generally parallel so as to be located on opposite sides of the hollow interior;
the plate conductors being arranged such that, in a second mode, current can be caused to flow in a circuit along the first plate conductor, through the first annular conductor to the second plate conductor, along the second plate conductor and through the second annular conductor to the first plate conductor;
the circuit including at least one third capacitor;
the circuit and the third capacitor being arranged to generate a second oscillating radio frequency field, at a resonant frequency dependent upon the shape and impedance of the first and second plate conductors;
the plate conductors having a width transverse to the length thereof which is sufficient to cause the second field to be substantially homogeneous within the hollow interior;
the first and second annular conductors and the first and second plate conductors being shaped and arranged and the first, second and third capacitors being arranged such that the first and second fields have the same resonant frequency and such that the first and second fields are substantially mutually orthogonal and arranged for co-operation with a magnetic field at right angles to the plane of the plate conductors.
Preferably, in order to effect tuning of the coil to the required frequency, the first and second capacitors are arranged relative to the conductors such that the capacitance thereof does not affect the resonant frequency of the second mode, and the third capacitor is arranged such that the capacitance thereof does not affect the resonant frequency of the first mode created by the first and second annular conductors.
Preferably the first capacitor is connected in series with the first annular conductor at a longitudinal split in one of the first and second plate conductors, wherein the second capacitor is connected in series with the second annular conductor at a longitudinal split in one of the first and second plate conductors and wherein at least one of the first and second plate conductors is split transversely with the at least one third capacitor connected therein in series. Thus in a simple arrangement, one of the plate conductors is split longitudinally and the same or the other plate conductor is split transversely. It is also possible for both to be split both longitudinally and transversely with capacitors provided between each of the splits. Other configurations between these two extremes can also be adopted depending upon requirements.
Preferably the first plate conductor is split longitudinally, the first capacitor is connected in series with the first annular conductor at the longitudinal split in the first plate conductor, the second capacitor is connected in series with the second annular conductor at the longitudinal split in the first plate conductor and one of the first and second plate conductors is split transversely with the at least one third capacitor connected therein in series. It will be appreciated that additional capacitors can be provided in series at additional splits in the plate conductors.
Preferably the first plate conductor is split both longitudinally and transversely so as to form four separate portions interconnected by said capacitors. In this arrangement, it is possible to add a tuning capacitor connected diagonally across the transverse and longitudinal splits which is adjustable to tune the fields so as to be more accurately orthogonal.
The apparatus includes a console for creating oscillating signals to supply to the coil for generating said first and second fields and a coupling or driving circuit for communicating said signals to the coil. The coil can be driven through the coupling circuit by mutual inductance with one or more driving coils. More preferably, the driving circuit includes a first cable communicating a part of the signal through capacitors, across a split in one of the annular conductors and a second cable communicating a part of the signal through capacitors, across a split in one of the plate conductors.
Preferably the driving circuit includes an adjustable impedance for matching the impedance of the coil to the impedance of the transmission lines and amplifiers.
Preferably for research uses rather than clinical testing, at least one of the capacitors in the annular rings and at least one of the capacitors on the plate conductors are adjustable for accurately tuning the resonant frequencies.
According to a second aspect the invention provides an apparatus for effecting magnetic resonance experiments on a sample comprising:
a magnet having a magnetic field extending in a unidirectional field direction through a space for receiving the sample;
a quadrature radio frequency field coil as defined above:
an MRI console for creating oscillating signals for supply to the coil for generating said first and second fields;
and a driving circuit for communicating said signals to the coil.
According to a third aspect the invention provides a method for effecting magnetic resonance experiments on a sample comprising:
providing a sample;
providing a magnet having a magnetic field extending in a unidirectional field direction through a space for receiving the sample;
providing a quadrature radio frequency field coil having:
a first annular conductor lying generally in a first plane at a first end of the coil;
a second annular conductor lying generally in a second plane at a second end of the coil;
the first and second planes being parallel and spaced so as to define a hollow interior of the coil into which the sample can be received;
the first annular conductor being shaped and arranged to surround an opening into the hollow interior;
the first annular conductor including at least one first capacitor connected therein in series;
the second annular conductor including at least one second capacitor connected therein in series;
a first plate conductor having a length extending across the annular conductors generally at right angles to the first and second planes so as to bridge the space therebetween;
a second plate conductor having a length extending across the annular conductors generally at right angles to the first and second planes so as to bridge the space therebetween;
the first and second plate conductors being arranged at diametrically opposed locations on the annular conductors and being generally parallel so as to be located on opposite sides of the hollow interior;
mounting the coil within the space arranged such that the magnetic field lies at right angles to the plane of the plate conductors;
inserting the sample into the hollow interior through one of the openings;
arranging the first and second annular conductors and the first and second capacitors and applying power thereto as an oscillating signal such that, in a first mode, current flows therein to generate a first oscillating radio frequency field, which is substantially homogeneous within the hollow interior, at a resonant frequency dependent upon the shape and impedance of the first and second annular conductors, the capacitors in series with them, and the mutual inductance therebetween;
arranging the first and second plate conductors and a third capacitor in series therein and applying power thereto as an oscillating signal such that, in a second mode, current flows in a circuit along the first plate conductor, through the first annular conductor to the second plate conductor, along the second plate conductor and through the second annular conductor to the first plate conductor to generate a second oscillating radio frequency field, at a resonant frequency dependent upon the shape and impedance of the first and second plate conductors and the impedance of the third capacitor;
arranging the plate conductors with a width transverse to the length thereof which is sufficient to cause the second field to be substantially homogeneous within the hollow interior;
and arranging the first and second annular conductors and the first and second plate conductors and arranging the first, second and third capacitors such that the first and second fields have the same resonant frequency and such that the first and second fields are substantially mutually orthogonal.